The proposed research will continue the work begun in 1967 aimed at realizing the quantitative potential of electron microscope autoradiography. I plan to determine the resolution of various useful isotopes such as I125, P32, P33, Ca45; to establish the effect both on resolution and sensitivity of heavy metal stains in the biological tissue; to calibrate procedures for processing negatively stained and shadowed specimens, frozen sections and thick sections for high voltage electron microscopy. Finally I want to develop new methods for analyzing autoradiograms and apply them to some current problems in cytology.